System and method using a packetized encoded bitstream for parallel compression and decompression

ABSTRACT

A system and method are disclosed for parallel compression and decompression of a bitstream. For compression, the bitstream is separated into a plurality of components, and the components are encoded using a compression algorithm. Packets are then constructed from the encoded components. At least one packet is associated with each encoded component and comprises header information and encoded data. The packets are combined into a packetized encoded bitstream. For decompression, the packets are separated from the packetized encoded bitstream using the header information. The packets are then decoded in parallel using a decompression algorithm to recover the encoded data. The plurality of components are reconstructed from the recovered encoded data and combined to-recover the bitstream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 09/099,742, filed Jun.18, 1998, entitled “System and Method Using a Packetized EncodedBitstream for Parallel Compression and Decompression.”

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of datacompression and decompression and, more particularly, to a system andmethod using a packetized encoded bitstream for parallel compression anddecompression.

BACKGROUND OF THE INVENTION

Compression and decompression algorithms are widely used for processingdigital bitstreams, especially where the bitstreams represent digitizedaudio, video, graphics, etc. One such algorithm is the a losslesscompression format (LCF) used in a number of products from SILICONGRAPHICS. The LCF format is related to a format presented in a NASA JPLpaper entitled “Algorithms for a Very High Speed Universal NoiselessCoding Module” by Robert F. Rice, Pen-Shu Yeh and Warner Miller (theRice algorithm). LCF is a lossless compression format that is relativelysimple to implement in hardware, thus there is a desire to use LCF inproducts that process large amounts of real-time data, including highperformance applications like HDTV image compression. However, a problemarises in that the performance needed to encode/decode, for example,HDTV resolution images at video rates is not generally possible usingthe conventional LCF architecture and application specific integratedcircuit (ASIC) technology. To overcome problems with performance, somecompression schemes try to process separate bitstreams using parallelprocessing paths. However, this typically is not helpful in improvingperformance within a single bitstream.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system and method aredisclosed that use a packetized encoded bitstream for parallelcompression and decompression and that provide significant advantagesover conventional compression and decompression schemes.

According to one aspect of the present invention, a method is providedfor parallel compression and decompression of a bitstream. Forcompression, the bitstream is separated into a plurality of components,and the components are encoded using a compression algorithm. Packetsare then constructed from the encoded components. At least one packet isassociated with each encoded component and comprises header informationand encoded data. The packets are combined into a packetized encodedbitstream. For decompression, the packets are separated from thepacketized encoded bitstream using the header information. The packetsare then decoded in parallel using a decompression algorithm to recoverthe encoded data. The plurality of components are reconstructed from therecovered encoded data and combined to recover the bitstream.

According to another aspect of the present invention, a system isprovided for parallel compression and decompression of a bitstream. Thesystem includes an encoder system and a decoder system. The encodersystem comprises a plurality of encode units that receive componentsseparated from a bitstream and encode the components using a compressionalgorithm. The encode units then construct packets from the encodedcomponents, where at least one packet is associated with each encodedcomponent and comprises header information and encoded data. The encodersystem also comprises a multiplexer coupled to the encode units. Themultiplexer combines the packets into a packetized encoded bitstream.The decoder system comprises a feeder that separates packets from thepacketized encoded bitstream. The decoder system also comprises aplurality of decode queues, and the feeder distributes the packets tothe decode queues. Further, the decoder system comprises a plurality ofdecode units each associated with one of the decode queues. The decodeunits decode packets using a decompression algorithm to recover theencoded data and to reconstruct the components. The decoder system thencomprises a demultiplexer coupled to the plurality of decode units thatcombines the plurality of components to recover the bitstream.

A technical advantage of the present invention is the ability toparallelize a compressed, variable-length bitstream in the decodingprocess. Markers or headers can be used in the bitstream to break up thedata into parallelizable tasks. This allows the location to beidentified, for example, of specific pixels or samples. Otherwise,without markers to assist in breaking up the data, the only way todetermine the location of specific pixels or samples in the bitstreamwould be by actually decoding the bitstream.

Another technical advantage of the present invention is theparallelization of LCF to achieve greater performance. Parallelizationcan be accomplished using a packetized bitstream having fixed-sized andvariable sized packets. This provides a lossless compression format thatis readily parallelizable.

A further technical advantage of the present invention is the breakingup of a single bitstream into fixed-size packets which encode a variablenumber of samples. Each packet can start with a fixed-length tag whichdirects each packet to a specific decoder unit. Since each packet has afixed-length with a tag field for directing, a distributor canefficiently send different packets to different decoder units which canthen process the packets in parallel. This scheme can attain parallelismat a low level so the encode/decode, for example, of -a single picturecan be parallelized without needing the complexity of trying to achieveparallelism by processing multiple pictures (streams) concurrently. Thedistribution of packets for decoding can be rapidly done because thepackets are fixed-length and the fixed size tag bits direct the packets.Also, the tag field overhead can be quite low. Further, the bufferingrequirements for both encode and decode can be quite reasonable which isimportant in achieving high performance through an on-chip memory.

An additional technical advantage of the present invention is thebreaking up of a single bitstream into variable-size packets with eachpacket encoding the same number of samples. Each packet can beindependent which means no other information is needed from otherpackets to decode the current packet. Each packet can start off with alength marker, giving the length of the packet. Since each packet has alength field and each packet is independent, a distributor canefficiently send different packets to different decode units which canprocess the packets in parallel. This scheme also can attain parallelismat a low level so the encode/decode, for example, of a single picturecan be parallelized without needing the complexity of trying to achieveparallelism by processing multiple pictures (streams) concurrently.Further, the bitstream is extremely flexible so the exact same bitstreamcan be used to achieve scaling amounts of parallelism. Any number ofprocessing units can be used on the same bitstream. This flexibilityallows a hardware implementation (and the number of processing units) tobe independent from the bitstream format. Also, this scheme has astraightforward input/output interface. For both encoding and decoding,the hardware can access pixels in simple scan-line order, left-to-right,top-to-bottom. Further, this scheme provides good load balancing. Eventhough the packets are variable size, each packet can represent(approximately) the same amount of work to encode/decode because eachpacket encodes the same number of samples. As a result, the distributionof packets to different processing units represents an equal sharing ofthe work. Finally, the buffering requirements for both encode and decodeare reasonable to help achieve high-performance through on chip memory.

Other technical advantages of the present invention should be readilyapparent from the drawings, specification and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 is a block diagram of one embodiment of a video processing cardusing LCF encoding and decoding;

FIG. 2 is a block diagram of one embodiment of a graphics system 30using LCF encoding and decoding;

FIG. 3 is a diagram of a conventional LCF encoded block;

FIG. 4 is a diagram of one embodiment of a fixed length packet used forparallelism according to the present invention;

FIG. 5 is a diagram of one embodiment of interleaving fixed-lengthpackets from multiple scan lines according to the present invention;

FIG. 6 is a diagram of one embodiment of a variable-length packet usedfor parallelism according to the present invention;

FIG. 7 is a block diagram of one embodiment of a parallel encoder systemaccording to the present invention;

FIG. 8 is a block diagram of one embodiment of a parallel decoder systemaccording to the present invention.

FIGS. 9A, 9B, 9C, 9D, 9E, 9F, and 9G are block diagrams of oneembodiment of the processing of variable-length packets by the feeder inthe parallel decode system of FIG. 8 according to the present invention;and

FIGS. 10A, 10B and 10C are block diagrams of one embodiment of theprocessing of fixed-length packets by the feeder in the parallel decodesystem of FIG. 8 according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of one embodiment of a video processing 10using lossless compression format (LCF) encoding and decoding. As shown,card 10 has a PCI bus interface 12 that can communicate across a PCI buswith a host system. Card 10 also has a video interface 14 that cancommunicate with a monitor or other video device. Both PCI interface 12and video interface 14 can access local memory 16 on card 10. Localmemory 16 provides storage and workspace for on-card processing. In theembodiment of FIG. 1, card 10 has two encode/decode sections coupled toaccess local memory 16. Card 10 has a discrete cosine transform (DCT)encode unit 18 and a DCT decode unit 20, and card 10 has an LCF encodeunit 22 and an LCF decode unit 24. In operation, LCF encode unit 22 andLCF decode unit 24 can implement parallel compression using a packetizedbitstream according to the present invention. Card 10 can provide highperformance video processing for the host system.

FIG. 2 is a block diagram of one embodiment of a graphics system 30using LCF encoding and decoding. As shown, graphics system 30 has agraphics engine 32 that can provide an output for display. Graphicsengine 32 is coupled to an LCF encode unit 34 and an LCF decode unit 36.Both LCF encode unit 34 and LCF decode unit 36 are coupled to hostsystem memory. In operation, LCF encode unit 34 and LCF decode unit 36can implement parallel compression using a packetized bitstreamaccording to the present invention. Graphics system 30 can provide highend graphics processing functions. FIG. 1 and FIG. 2 provide examples ofhardware applications in which parallel compression using a packetizedbitstream can be implemented. It should be understood that various otherhardware and software applications also could benefit from such acompression scheme.

In this context, the “block” is generally used herein to refer to agroup of several consecutive samples which are encoded together usingthe same compression mode. The term “packet” is generally used to mean afixed-length or variable-length set of bits that contains some portionof a block. In the case of fixed-length packets, a packet may contain afraction of a block, one block, or multiple blocks. In the case of avariable-length packet, the packet often contains multiple blocks. Theterm “multi-pack” generally refers to the packets needed to encode a V×Hregion where, for example, data is compressed using V×H rectangularregions of pixels, left-to-right, top-to-bottom. (V is the number oflines, and H is the number of pixels per line.) The term “unit”generally refers to a processing pipeline for decoding or encoding LCFdata in serial fashion. Conventional LCF consists of a single decodeunit and a single encode unit. The present invention achievesparallelism and uses multiple decode and encode units. The term “system”generally refers to the logic used for either decoding or encoding abitstream, including input/output interfaces and multiple units. Thedecode system and encode system may both have multiple units.

Conventional LCF

The following is a general description of LCF which should provide ahelpful context. LCF involves encoding blocks of deltas. For LCF, thenumber of deltas in a block is typically either 15 or 16. These deltasare obtained by performing Differential Pulse Code Modulation (DPCM) onthe samples. The previous (left) horizontal sample (x_(i-l)) is used asa predictor for the current sample (x_(i)). The equation is:Δ=X _(i) −X _(i-l)At the start of each scan line, the predictor is reset. The first sampleof the line is used as the reset predictor value.

FIG. 3 is a diagram of a conventional LCF encoded block 38. As shown,each encoded block 38-consists of two parts: a mode field and theencoded deltas. The mode field is a fixed number of bits (e.g., 3 bitsfor 8 bit samples and 4 bits for 9, 10, 11, or 12-bit samples). Theencoded deltas are a variable number of bits and represent the actualdata encoded using the specified compression mode.

There are several shortcomings in conventional LCF that can makeparallelism difficult. The blocks are dependent on each other. The lastsample from the previous block needs to be decoded in order for it to beused as the predictor for the first sample in the current block. As aresult, parallelism at a block granularity is difficult. The predictoris reset only at each line boundary so the unit of independence for LCFis a line which, for example, for HDTV resolution could be up to 1920samples. From a hardware perspective, using on-chip memory to buffer anHDTV line per processing unit is very expensive. In addition to thebuffering issue, trying to distribute the independent segments of dataefficiently is a problem. There are no markers or alignments (exceptdouble-word alignment at the end of a picture) in the LCF bitstream.Without markers, it is not possible to determine where one encoded lineends and the next one begins unless the bitstream is actually decodedand a count of decoded samples is made.

Fixed-Length Packet

FIG. 4 is a diagram of one embodiment of a fixed-length packet 40 usedfor parallelism according to the present invention. The bitstream can bebroken into fixed-size packets 40 which encode a variable number ofsamples. Each packet 40 can start with a fixed-length tag which directsthe packet to a specific decoder. Since each packet is a fixed-lengthand has a tag for direction, a distributor can efficiently senddifferent packets to different decode units which can then process thepackets in parallel. As shown in FIG. 4, the Tag can be a fixed-lengthfield used to direct the packet to a specific decoder unit. For example,if the Tag were “1”, the packet would get directed to decoder unit “1”.If a V×H region is encoded for the multi-pack, then the number of tagbits used is given by ceil (log₂(V)). In packet 40, the EncData fieldcan contain a fixed-length of encoded data that has been encoded usingregular LCF compression. Because packet 40 is fixed-length and LCFencoded data is variable-length, as the encoded data exceeds a packetboundary, the excess encoded data can be used to start a new packet 40.

An important point to note is that, in this embodiment, the taginformation is embedded and distributed into the bitstream instead ofhaving a table of information as header for the bitstream. A table ofinformation would require both the encoder and decoder to buffer thisinformation. With the embedded and distributed tag information, thebuffering requirements for this information is minimal.

FIG. 5 is a diagram of one embodiment of interleaving fixed-lengthpackets from multiple scan lines according to the present invention. Asmentioned above, the unit of independence for LCF is a scan line. Thefixed-length packet scheme for parallelization can take advantage ofthis by interleaving packets from multiple scan lines. For example, ifparallelism by four is desired, four consecutive scan lines can beencoded and interleaved. As shown, an uncompressed rectangular region 42can be compressed to form a compressed region 44. The compressed region44 can then be formed into a bitstream 46 with packet interleaving. Inthis example, two tag bits could be used to distribute to four decoderunits. Tag 0 could be used for packets that encode data from lines 0, 4,8, etc. Tag 1 could be used for lines 1, 5, 9, etc. Tag 2 could be usedfor lines 2, 6, 10, etc., and Tag 3 could be used for lines 3, 7, 11,etc. Data is then encoded using a rectangular region of pixels,left-to-right, top-to-bottom. For example, if 32 pixels are encoded at atime and with parallelism by four, a 4×32 region would be encoded at atime. For each of the four different lines, a different tag could beused, and packets could be interleaved in a round-robin scheme. As shownin FIG. 5, the four segments (portions of lines) could have differentcompression ratios so the four compressed segments could each result ina different number of fixed-length packets. If this is the case, thenafter doing round-robin interleaving and running out of certainsegments, the remaining segments would be interleaved in round-robinfashion, as shown. After interleaving all the packets for a region, anycompressed data that is left over and that is not sufficient to create anew complete packet can be held over and combined with the data for thenext 4×32 region to create whole packets.

Through the tags, an imbalance in the compression ratios does notcomplicate or break down the parallelization scheme. The decode systemis not expecting nor does it know about any particular packetinterleaving scheme. For the decoder system, packets can be distributedto different decoders solely through the tags. As a result, theparallelization scheme using these fixed-length packets works regardlessof the compression ratios of the segments. Within a multi-pack, more ofthe packets may be directed to some decoder units than other units, butthe tag scheme can ensure that the packets are efficiently steered tothe correct decoders.

With respect to boundary conditions, a new packet can be started foreach new scan line. If the last packet for a scan line only fills up apartial packet, then the packet can be padded with enough zero-valuedbits to create a whole packet. The decoder can keep track of the numberof samples per scan line to discard the zero-value bits, if necessary.

Data can be encoded using V×H regions. If the number of pixels in a scanline does not divide evenly by H, then the last pixel of a scan line canbe replicated so that the number of pixels encoded in a line can be amultiple of H. If the number of lines in the picture does not divideevenly by V, then the last group of lines in the image can beparallelized by the number of lines left. For example, if an image has31 lines and V is 4, then the last 3 lines can be encoded with onlyparallelism by 3. At the end of a compressed image, zero-value bits canbe padded in to achieve double-word alignment for the compressed image.The decoder system then should be able to discard this zero-bit padding,if necessary.

With respect to component interleaving, fixed-length packetization canbe can be done on a V×H rectangular region of pixels. Pixels generallyconsist of one or more components. The tag bits can be used todistribute the packets to different decoders, and the tag bits representdifferent scan lines. Within each scan line, the data can be encoded byinterleaving blocks of components. This interleaving scheme can be thesame one used for conventional LCF. Having the packetization scheme usethe same interleaving pattern helps to simplify the logic that will beused to encode/decode both LCF and packetized LCF. For example, whenencoding pixels which have four fully sampled components (RGBA,4:4:.4:4:), there would be block of R, then a block of G, then a blockof B, and finally a block of A before moving on to the next set ofpixels. It should be noted that when encoding each block, the resultingencoded block could form a fraction of a packet, one packet, or multiplepackets. The interleaving schemes can be as set forth in the followingtable.

TABLE 1 Format Encode Decode RGBA (4:4:4:4) R, G, B, A R, G, B, A RGB(4:4:4) R, G, B R, G, B— YCrCb (4:2:2) Cb, Y₀, Cr, Y₁ Cb, Y₀, Cr, Y₁YCrCbA Cb, Y₀, A₀, Cr, Y₁, A₁ Cb, Y₀, A₀, — Cr, Y₁, A₁, — YAYA (4:0:0:4)Y₀, A₀, Y₁, A₁ Y₀, A₀, Y₁, A₁ YYYY (4:0:0) Y₀, Y₁, Y₂, Y₃ Y₀, Y₁, Y₂, Y₃In TABLE 1, the encode column shows how the encoded blocks can beinterleaved, and the decode column shows what an output pixel can looklike. A “—” in the table means that nothing is stuffed into thatcomponent position for that particular format.Variable-Length Packet

In contrast with the fixed-length packet, FIG. 6 is a diagram of oneembodiment of a variable-length packet 48 used for parallelism accordingto the present invention. The bitstream can be broken into variable-sizepackets 48 with each packet encoding the same number of samples. Eachpacket can be independent such that no other information is needed aboutother packets to decode the current packet. In general, each packet 48can start off with a length marker giving the length of the packet andcan be byte aligned. The length field and byte alignment allows adistributor to determine the end of a packet rapidly and send a packetof data efficiently to a decoding unit. Thus, as shown in FIG. 6, theSize field can tell the total size of the packet in bytes. The Predfield can be a starting predictor to use for the first sample of thepacket. This predictor value can be encoded. The EncData field can bevariable-length data encoded using regular LCF compression. For example,if each packet contains N samples, then N−1 of the deltas are encoded inthis section. Only N−1 deltas instead of N need to be encoded since thefirst sample of the packet can be held in the Pred field. The Alignfield can be any padding bits needed to byte align the packet.

Analogous to the fixed-length packets, this scheme allows the sizeinformation to be embedded and distributed into the bitstream instead ofhaving a table of size information as a header for the bitstream.Otherwise, a table of size information would require both the encoderand decoder to buffer this information. With the embedded anddistributed size information, the buffering requirements for sizeinformation can be minimal.

With respect to boundary conditions, each packet can represent N samplesand a new packet can be started for each new scan line. If the number ofsamples in a scan line is not evenly divisible by N, then the lastpacket for the line can represent the last sample of the line so thatthe last packet contains N samples. The decoder can then discardreplications, if necessary. Further, at the end of a compressed image,zero-value bits can be padded in to achieve double-word alignment forthe compressed image.

With respect to interleaving, the N samples of a packet can representvalues from a single component. The input into the encoder system can bepixels, and the output from the decoder system can be pixels. Pixelsconsist of one or more components. To make it easier to assemble pixelson the decode side, the components can be interleaved. For example, whenencoding N pixels with each pixel having 4 components (RGBA,4:4:4:4),there would be a packet of G, then a packet of B, and finally a packetof A before moving on to the next N pixels. Each packet contains Nsamples of a particular component. The interleaving schemes could be asfollows:

TABLE 2 Format Encode Decode RGBA (4:4:4:4) R, G, B, A R, G, B, A RGB(4:4:4) R, G, B R, G, B, — YCrCb (4:2:2:4) Cb, Y₀, Cr, Y₁ Cb, Y₀, Cr, Y₁YCrCbA (4:2:2:4) Cb, Y₀, A₀, Cr, Y₁, A₁ Cb, Y₀, A₀, Cr, Y₁, A₁ YAYA(4:0:0:4) Y₀, A₀, Y₁, A Y₀, A₀, Y₁, A₁ YYYY (4:0:0) Y₀, Y₁, Y₂, Y₃ Y₀,Y₁, Y₂, Y₃ In TABLE 2, the encode column shows how the packets can beinterleaved, and the decode column shows what an output pixel can looklike. A “—” means that nothing is placed into that component positionfor that particular format.Parallel Encoder System

FIG. 7 is a block diagram of one embodiment of a parallel encodersystem, indicated generally at 50, according to the present invention.Encoder system 50 comprises a pixel buffer 52 that feeds a plurality ofencoder units 54. Encode units 54, in turn, feed a multiplexer 56 whichbuilds an encoded bitstream. Encode units 54 can be associated withprocessing and producing packets for particular lines or components, asshown. Encoder system 50 can be implemented to parallelize the encodedbitstream using fixed-length packets or using variable-length packets.For fixed-length packets, each encode unit 54 can be associated withparticular lines (e.g., 0, N, 2N, . . . ). For variable-length packets,each encode unit 54 can be associated with particular components andcomponent types (e.g., 0, N, 2N, . . . ).

For variable-length packets, encoder system 50 can be parallelized byhaving multiple encode units 54 working on different packets at the sametime. Each encode unit 54 can work on a different packet of N samples.To allow for parallel encoding, the incoming pixel buffer 52 can allowfor as many simultaneously readers as there are parallel encode units54. Also, the packets need to be processed in the order necessary tooutput the proper interleaved-components format in the encodedbitstream.

For fixed-length packets, encoder system 50 again can be parallelized byhaving multiple encoder units 54 working on different packets at thesame time. In this scheme, the pixel buffer 52 holds a V×H region ofpixels at a time. Each encode unit 54 can work on one of the V differentsegments (portion of a line with H pixels). As above, to allow forparallel encoding, the incoming pixel buffer 52 needs to allow for asmany simultaneous readers as there are parallel encode units 54. Also,the packets must be processed in the packet interleaving patternspecified above. All of the complete packets that can be made from theV×H region of pixels in pixel buffer 52 are output in the encodedbitstream before working on the next V×H region of pixels.

Parallel Decoder System

FIG. 8 is a block diagram of one embodiment of a parallel decodersystem, indicated generally at 60, according to the present invention.Encoder system 60 comprises a feeder (distributor) 62 that feeds aplurality of decode queues (FIFO) 64. A plurality of decode units 66 areeach associated with one of decode queues 65. Decode units 64, in turn,feed a pixel sync 68 which provides decoded pixel outputs. Decode units64 can be associated with processing packets for particular interleavedlines, as shown. Decoder system 60 can be implemented to process abitstream parallelized using fixed-length packets or usingvariable-length packets. As above, for fixed-length packets, each decodequeue 64 and decode unit 66 pair can be associated with particular lines(e.g., 0, N, 2N, . . . ). For variable-length packets, each decode queue64 and decode unit 66 pair can be associated with particular componentsand component types (e.g., 0, N, 2N, . . . ).

Decode of Variable-Length Packets

FIGS. 9A, 9B, 9C, 9D, 9E, 9F, and 9G are block diagrams of oneembodiment of the processing of variable-length packets by feeder 62 inparallel decode system 60 according to the present invention. Since eachvariable-length packet is independent, the decoding process can beparallelized. Feeder 62 takes in the compressed encoded bitstream anddistributes different packets to different decode queues 64 and decodeunits 66. In the embodiment of FIGS. 9A–9G, the compressed bitstream isreceived by an input queue (FIFO) 70. A multiplexer 72 pulls bits frominput queue 70 and feeds a register 74. The contents of register 74 areprovided to a left shift logic 76 and to a demultiplexer 78.Demultiplexer 78 then distributes the bits to the appropriate decodequeue 64.

In this scheme, because each packet has a length field and can bebyte-aligned (see FIG. 6), feeder 62 can efficiently distribute thepackets. Having feeder 62 feed multiples of four decode units 66 (i.e.,4,8,12, etc.) can make implementation simpler since pixels can becreated based on interleaving four components (or less) as shown abovein TABLE 2. An assumption for feeder 62 is that feeder 62 can supplydata to decode queues 64 and decode units 66 as fast as or faster thanthe rate which the combined number of decode queues 64 and decode units66 can consume data. To decode multiple packets in a concurrent fashion,decode queues 64 are used so feeder 62 can distribute packets to otherdecoder units 66 while a decode unit 66 can be still busy processingpackets it has received. Each packet that a decoder unit 66 receives canbe decoded into N samples for a single component. As is explained abovewith respect to interleaving, the components are interleaved to formpixels. The pixel maker logic of pixel sync 68 can take the componentsamples output from decode units 66 and can interleave them to formpixels according to the specified sampling format.

One key to parallel decoding can be the ability for feeder 62 to sendpackets rapidly to decoder units 66. FIGS. 9A–9G illustrate oneembodiment of how feeder 62 can distribute packets. In particular,FI-GUREs 9A–9G show how two example packets (PACKET 0 and PACKET 1) flowthrough feeder 62. Byte alignment can allow for a simple, efficientprocess where the depth-of multiplexing can be reduced, and the lengthfield can be quickly parsed. Being able to quickly distribute packetscan depend on being able to pick out the length field of a packetefficiently. Picking out the length field would get complicated if thelength field could get broken up into two entries of input queue 70. Insuch a case, determining the length of a packet could take two cycles.

With byte alignment and assuming the queue width can be some integralnumber of bytes greater than one, if a packet ends in the middle ofqueue entry, then the smallest amount of space left in the queue entryfor the next packet can be a byte. This means that as long as the lengthfield is eight bits or less, the length field can be guaranteed to notbe divided between two queue entries. Eight bits allows the length fieldto represent packets up to 256 bytes. One disadvantage of byte alignmentcan be the extra padding which could be required for alignment. However,this overhead can be typically small on average and only four bits arewasted for alignment on average. However, an advantage is that thelength field can be in bytes instead of bits, thus the length field canbe three bits shorter with byte alignment than without this alignment.

In FIGS. 9A–9G, PACKET 0 is shown to start aligned to a queue entry.Although PACKET 1 starts in the middle of a queue entry, by the timePACKET 1 needs to be processed, its length field has been left-justifiedin register 74 (FIG. 9E). This means that as long as the length field iseight bits or less, feeder 62 can easily pick out the length field byexamining just the left-most bits in register 74. Remember that for avariable-length packet, the first field can be the length (see FIG. 6).This simplifies the logic since finding the length can be simply lookingat these left-most bits. FIGS. 9B and 9E show how the length field endsup in the left-most bits of register 74. Another thing to note is thewasted space which can occur at the start or end of transferring apacket. Wasted space translates into wasted bandwidth. For PACKET 0,FIG. 9C shows no space is wasted to send the first portion of the packetbecause the packet started aligned to the queue entry. Also, the secondportion wastes no space because it takes up the entire entry as shown inFIG. 9D. However, in FIG. 9E, it is shown that the last portion forPACKET 0 wastes space because it ends in the middle of a queue entry.Wasted space results because only a portion of data gets sent to adecode unit 66. This means if a queue entry contains portions of twopackets, it will take two transfers to distribute this data with eachtransfer having wasted space. For PACKET 1, space is wasted for sendingthe first portion (FIG. 9F) because the packet does not start aligned toa queue entry. Also, space can be wasted for sending the last chunk(FIG. 9G) because the packet does not end aligned to a FIFO entry.

With respect to packet size, there are several things to consider whentrying to determine a good packet size. In order to parallelize thebitstream, overhead is incurred for the size, predictor, and bytealignment fields. This overhead makes the compression ratio somewhatworse, thus making the packet longer helps to amortize the overheadcosts. Another reason to increase the packet size can be to increase theparallelism realized. Given that input queue 70 coming into feeder 62 issome fix length and only one queue entry can be pulled out per cycle,the maximum performance that can be achieved is when the decoder systemthroughput rate is equal to the number of samples in a queue entry percycle. For example, if, on average, a queue entry has 16 compressedsamples, then the maximum decoder system throughput would be 16 samplesper cycle. However, as mentioned above, feeder 62 may haveinefficiencies where space might get wasted when sending either thefirst and/or last portion of a packet because of misalignment with thequeue entry. This decreases the amount of parallelism realized. However,this problem can be offset by making the size of a packer bigger so thatthe wasted space (bandwidth) from sending the first and last portion ofa packet can be small compared to the overall packet size. A portion ofa packet which is not the first and/or last portion occupies an entirequeue entry and can be sent with no wasted space (FIG. 9D). However,packets can be made too big. As FIG. 8 shows, each decode unit 66 needsto be able to buffer a packet. As the packet size grows, the amountwhich must get buffered by each decoder unit 66. This type of on-chipbuffering can be relatively expensive.

With respect to sustainable parallelism, misalignments for the first andlast portion of data transmitted by feeder 62 can limit achievableparallelism. For example, if 32 samples per packet, 8 bits per sample,and 2:1 compression ratio are assumed, then the LCF compressed sizewould be 128 bits for 32 samples. It can also be assumed that the inputto the decoder system can be 64 bits wide. Then, in an ideal case, thedecode throughput rate would be 32 samples in two cycles. This can bethe fastest rate possible given that the input is limited to 64bits/cycle. This 32 samples in two cycles results in an idealsustainable parallelization of 16 (i.e., 32/2). Thus, up to 16 unitscould be kept fully busy in this scenario. Actual sustainableparallelization means that the combined rate of data consumption by alldecoder units 66 is less than or equal to the rate which feeder 62 cansend out packet data. In reality, the ideal sustainable parallelizationis not achievable because of the inefficiencies of feeder 62 resultingfrom misalignments with the queue entries.

Sustainable parallelization can be calculated by taking the number ofsamples in a packet and dividing it by the number of cycles to dispatchthe packet. For example, if a packet contains 32 samples and it takesthe feeder four cycles to dispatch this packet, then the sustainableparallelism can be 8 (i.e., 32/4). Thus, eight decode units 66 could bekept fully busy in this example.

For one implementation, the variable-length packets contain 32 samplesor 64 samples. As a reminder, conventional LCF uses blocks of 15 to 16samples (see FIG. 3). The mode field can be 3 bits for 8-bit samples and4 bits for 9 to 12-bit samples. Further, as a reminder, avariable-length packet as set forth above includes three additionalfields (Size, Pred and Align) in addition to the conventional LCF block(EncData) (see FIG. 6). Thus, for 32-sample packets, the EncData sectionwill contain 31 encoded samples (63 encoded samples for 64-samplepackets). Conventional LCF uses blocks of 15 to 16 deltas, and theseblocks are encoded using a look-up table. To re-use these look-uptables, 31 samples can be encoded as 1 block of 16 deltas followed by 1block of 15 deltas. For 64 sample packets, 63 samples can be encoded inthe EncData section as 3 blocks of 16 deltas followed by 1 block of 15deltas. The following table provides example lengths for the fields inthese two cases.

TABLE 3 32-sample Size: 6 bits packet Pred: 8–12 bits depending onsample Precision EncData: 37–380 bits depending on compression ratioAlign: 0–7 bits depending on padding needed 64-sample Size: 7 bitspacket Pred. 8–12 bits depending on sample Precision EncData: 75–772bits depending on compression ratio Align: 0–7 bits depending in paddingneeded

With respect to buffering requirements, buffering can be based on asimple-buffering scheme. Everything can be double buffered. There may bemore efficient ways of using less buffering, but this provides a generalidea about the buffer sizes. An important point to remember is that forcompressed data, the buffer size needs to be enough to support the worstcase compression ratio. In the worst case, the samples are unencodedwith the overhead of the mode bits for the block(s) and the overhead ofthe size, predictor, and alignment fields for the packet. The followingtable provides example buffering capacity.

TABLE 4 32-sample Decode Input - 128 bytes/(decode unit) packet Output -128 bytes/(decode unit) Encode Pixel Buffer - 256 bytes Delta Buffering(within each Encode Unit) 128 bytes/(encode unit) Output Buffering(within each Encode Unit) - 128 bytes/(encode unit) 64-sample DecodeInput - 256 bytes/(decode unit) packet Output - 256 bytes/(decode unit)Encode Pixel Buffer - 512 bytes Delta Buffering (within each EncoderUnit) - 256 bytes/(encode unit) Output Buffering (within each EncoderUnit) - 256 bytes/(encode unit)

The performance of conventional LCF and LCF after parallel compressionusing variable-length packetized bitstream can be compared. Thefollowing tables give such comparisons for 32-sample and 64-samplevariable-length packets. The numbers in the tables are estimates basedon assumptions and can be confirmed with actual data from images. The“Parallelized LCF” and “LCF” columns show the size of the compresseddata in images. The “%Overhead” column shows the overhead caused by theextra data in Parallelized LCF. This overhead can be calculated as apercentage: (Par_LCF_size−LCF_size)/LCF-size*100%. In the tables, thefirst number in parentheses in the “Parallelized LCF” column shows thefewest number of cycles needed to dispatch a packet. For these tables,it can be assumed that the feeder datapath is 64 bits. This means theinput queue width, register, and left shifter widths are all 64 bits.Thus, the number of cycles needed to dispatch can be calculated simplyas: ceil(data_size/64). This number can be the better case because inthe worse case one extra cycle can be needed due to queue entrymisalignment. The second number in parentheses is the best sustainableparallelism for the specific case. This number can be equal to thenumber of components in a packet divided by the number of cycles todispatch (first number in parentheses).

Going through the rows of the tables, “uncompressed” refers to the casewhere the data can be represented unencoded (uncompressed). “Best case”is the smallest packet which can be achieved. Remember packetizationincurs the overhead for the size, predictor, and alignment fields. Thus,in the best case, the EncData field requires just 1 bit/sample plus themode field overhead per block. The alignment field can be the number ofbits needed to get the best-case packet to be a multiple of 8 bits. Forthe “average case”, the LCF column assumes a compression ratio of 2:1versus uncompressed. For Parallelized LCF, a compression ratio of 2:1versus uncompressed is assumed for the data in the EncData field. Onceagain, Parallelized LCF incurs overhead for the size, predictor, andalignment field. For average case, the alignment field is assumed to befour bits. Lastly, “worst case” occurs when the samples need to bepacked unencoded plus the overhead of the mode field per block.Parallelized LCF incurs the additional overhead of the size, predictor,and alignment fields. The alignment field can be the number of bitsneeded to get the worst-case packet to be a multiple of 8 bits.

TABLE 5 32-SAMPLE PACKETS 8-BIT COMPONENTS Parallelized Case LCF LCF %Overhead Uncompressed — 256 Best  56(1, 32) 38 48% Average 142(3, 10)128 11% Worst 272(5, 6)  262 4%

TABLE 6 10-BIT COMPONENTS Parallelized Case LCF LCF % OverheadUncompressed — 320 Best  56(1, 32) 40 40% Average 175(3, 10) 160 10%Worst 336(6, 5)  328 3%

TABLE 7 12-BIT COMPONENTS Parallelized Case LCF LCF % OverheadUncompressed — 384 Best  64(1, 32) 40 60% Average 208(4, 8)  192 9%Worst 400(7, 4)  392 2%

TABLE 8 64-SAMPLE PACKETS 8-BIT COMPONENTS Parallelized Case LCF LCF %Overhead Uncompressed — 512 Best  96(2, 32) 76 26% Average 271(5, 12)256 6% Worst 544(9, 7)  524 4%

TABLE 9 10-BIT COMPONENTS Parallelized Case LCF LCF % OverheadUncompressed — 640 Best  96(2, 32) 80 20% Average 336(6, 10) 320 5%Worst 664(11, 5) 656 1%

TABLE 10 12-BIT COMPONENTS Parallelized Case LCF LCF % OverheadUncompressed — 768 Best 104(2, 32) 80 30% Average 401(7, 9)  384 5%Worst 792(13, 4) 784 1%Decode of Fixed-Length Packets

FIGS. 10A, 10B and 10C are block diagrams of one embodiment of theprocessing of fixed-length packets by feeder 62 of in parallel decodesystem 60 (of FIG. 8) according to the present invention. The decodingprocess can be easily parallelized since each packet is a fixed length,has tag bits that provide a target decoder unit, and is independent ofother packets sent to different decoders. In the embodiment discussedabove, packets with different tags represent encoded data from differentlines. Two different lines are independent and thus can be decoded inparallel. Data from the same line can be dependent and goes to the samedecoder because these packets have the same tag bits. Also, the lengthof the packet can be optimized to be some multiple of the width of theinput buffer coming into decoder system 60. Having the packet size be amultiple of the buffer width allows the packets to be distributedefficiently.

Feeder 62 can take the compressed bitstream into an input queue (FIFO)90 as shown in FIG. 10A. Feeder 62 can then distributes differentpackets according to the tags as shown in FIGS. 10A and 10B. Oneassumption for feeder 62 is that it can supply data to decode units 66as fast as or faster than the rate which the combined number of decoderunits 66 can consume data. To decode multiple packets in a concurrentfashion, decode queues 64 are needed so feeder 62 can distribute packetsto other decode units 66 while a unit can be still busy processing thepacket(s) it has received. Each decode unit 66 receives packets whichdecodes in to a different scan line from the packets sent to other units66. After feeder 62 has distributed the packets of a multi-pack, eachdecode unit 66 will output H pixels for one of the V lines in the V×Hregion. The pixel sync logic 68 makes sure that the H pixels from eachdecode unit 66 are output in the correct order to reconstruct the V×Hrectangular region.

One of the keys to parallel decoding can be the ability for feeder 62 tosend packets rapidly to decode units 66. FIGS. 10A–10C show how feeder62 can distribute fixed-length packets. To simplify the diagram, FIGS.10A–10C show that each packet can be the same size as the width of inputqueue 90. However, just by having the packet length be some multiple ofthe input queue width, the logic can be simple. The idea is that feeder62 can extract the tag from the start of each packet. Since each packetis a fixed width and the tag is a fixed width, the tag can be easilyextracted. Thus, the tag can be stripped from the packet, and theremaining EncData field of the packet can be shipped to the properdecode unit 66 based upon the tag bits. Consequently, the feeder logiccan be relatively straightforward.

There are several factors to consider when trying to determine a goodpacket size for the fixed-length packets. This scheme for parallelizedLCF incurs the overhead from the tag filed while conventional LCF onlyhas the EncData section. This overhead makes the compression ratio worseso that making the packet longer helps to amortize the overhead costs.However, packets can not be made arbitrarily large due to bufferingrequirements. As shown in FIG. 8, each decode unit 66 needs to be ableto buffer some data. This buffering is needed so that while feeder 62 isdistributing data to the other decoder units 66, the current decode unit66 has data in its decode queue 64 to work on. For a multi pack, somedecoder units 66 might get more packets than other units 66 because ofvariation in compression ratios. In an effort to balance out thesevariations, the queue 64 in front of each decode unit 66 can be able tostore some multiple number of packets.

For one implementation, the fixed-length packets contain 64 bits, and,as a reminder, the fixed-length LCF packet is shown in FIG. 3. The queuewidth into the decode system can be assumed also to be 64 bits. Also,the current level of parallelism desired can be 4 (or possibly 8), andthe multi-pack region would be V=4 (or 8) and H=32. The following tableprovides example lengths for the fields in these cases.

TABLE 11 Parallelism Tag: 2 bits by 4 EncData: 62 bits Tag Overhead: 3%Parallelism Tag: 3 bits by 8 EncData: 61 bits Tag Overhead: 5%

Buffering for this example can be based on a simple double-bufferingscheme. Buffering multiple packets per decode unit can be required tobalance out the different compression ratios between the packets goingto different decode units. Experimental data has shown that buffering 4packets per decode unit provides high efficiency in all test cases.Regardless of compression ratio, a decode unit will generally output thesame number of samples for a given number of cycles. Variablecompression ratios means that some decode units will get more packetsthan others. Because all the units decode roughly the same number ofsamples in a given number of cycles, units which get more packets from amulti-pack consume their data faster than units which get less data.This works in favor for allowing the input buffering per decoder unit tobe fairly small. Finally, an important point to remember is that, whenhandling compressed data, the buffer size needs to be big enough tosupport the worst case compression ratio. The following table showsexample buffering for 64-bit packets.

TABLE 12 Decode Input - 64 bytes/(decode unit) Output - 128bytes/(decode unit) Encode Pixel Buffer - 512 bytes Delta Buffering(within each Encode Unit) - 128 bytes/(encode unit) Output Buffering(within each Encode Unit) - 128 bytes/(encode unit)

Although the present invention has been described in detail, it can beunderstood that various changes, substitutions and alterations can bemade thereto without departing from the sphere and scope ofthe inventionas defined by the appended claims.

1. A system for parallel compression and decompression of a bitstream,comprising: an encoder system comprising: a plurality of encode unitsoperable to receive components separated from a bitstream and to encodethe components using a compression algorithm; the encode units furtheroperable to construct packets from the encoded components, where atleast one packet is associated with each encoded component and the atleast one packet comprises header information and encoded data; and amultiplexer coupled to the encode units, the multiplexer operable tocombine the packets into a packetized encoded bitstream; and a decodersystem comprising; a feeder operable to separate packets from thepacketized encoded bitstream; a plurality of decode queues, the feederfurther operable to distribute the packets to the decode queues; aplurality of decode units each associated with one of the decode queues,the decode units operable to decode packets using a decompressionalgorithm to recover the encoded data and to reconstruct the components;and a pixel sync coupled to the plurality of decode units the pixel syncoperable to combine the plurality of components to recover thebitstream.
 2. The system of claim 1, wherein the bitstream is adigitized graphics or video frame.
 3. The system of claim 2, wherein theplurality of components comprise separate lines of the digitizedgraphics or video frame.
 4. The system of claim 3, wherein the encodeunits use a lossless compression format for the compression algorithm.5. The system of claim 3, wherein the, encode units constructvariable-length packets.
 6. The system of claim 5, wherein the headerinformation of the at least one packet comprises a size, a predictor andan alignment.
 7. The system of claim 6, wherein the header informationof the at least one packet comprises a tag.
 8. The system of claim 7,wherein the feeder distributes the packets to separate decode queuesbased upon the tag.
 9. The system of claim 7, wherein the feedercomprises an input queue operable to receive the packetized encodedbitstream.
 10. The system of claim 1, wherein the demultiplexer combinesthe plurality of components to recover the digitized graphics or videoframe for display.